In a large electric generator stator coils are retained in slots within the stator, the coils are in turn held in place by stator wedges. The stator wedges are inserted within a pair of opposed parallel grooves on either side of the stator slot. Ripple springs are inserted between the stator wedges and the coils to maintain a positive load on the coils to thereby maintain a consistent pressure or preload on the stator coil of the stator. It is important to monitor the load on this ripple spring to insure that the pressure on the stator coil remains constant. Should the stator coils not be held to the proper tightness, insulation failure due to vibration caused by mechanical and magnetic loading can occur.
There are various methods and apparatuses to test the tightness of stator wedges and ripple springs. One method is to measure the load on the stator coil's ripple spring by first drilling several holes along the length of the stator wedge. At each of these holes, the measurement of the height of the ripple spring is taken. Once all measurements have been recorded and analyzed, the maximum height of the ripple spring and the load on the ripple spring can be determined. One drawback to this method is that for a typical generator having 405 stator slots with three spaced wedges along each slot, each wedge having seven holes therein, 8505 ripple spring readings must be made and recorded.
Application Ser. No. 403,419, filed on Sept. 1, 1989, discloses an automated prober for testing generator stator wedge tightness with the rotor in place; the above referenced application being assigned to the present assignee and incorporated herein by reference. In this apparatus, a probe is inserted through the stator wedge holes and contacts the peak of the ripple spring. An apparatus on a carriage measures the extension of the probe which is successfully aligned with several holes in the stator wedge to determine the degree of compression of the spring. Additionally, this carriage is insertable in the narrow gap between the rotor and the stator and selectively positionable adjacent each hole in the series of holes through the stator wedges.
Stator wedge tightness can be measured manually by pressing down on the slot wedge to determine the ripple spring load. However, presently known methods require the removal of the rotor prior to such testing, which is a lengthy and costly procedure. Several other prior art methods are available to test the tightness of stator wedges and ripple springs, such as tapping the stator wedge and feeling or listening to the response therefrom to make a judgement on stator wedge tightness. This method depends on the experience of the technician doing the measurement.
While the above methods are quite acceptable in determining the stator wedge and ripple spring tightness, it is desirable to have a system which does not require exact positional accuracy in order to insert a probe through a myriad of holes in the stator wedges. Furthermore, it is still desirable to have an apparatus which is insertable within the generator while the rotor is still in place.